Kenwood KR-4010 (late 70's vintage) with signal to IF amp but no output

When I obtained the unit, the ouput protection relays chattered for the first couple of seconds after power up and then the output cut-out completely. Now there is no output at power on. With a scope I see the RF signal at the input to IC1 for both channels and see the signal at the left channel output of IC1( pin 6) but the right channel output of IC1 (pin 10) is sitting at -34 Vdc. I am looking for ideas on best actions to take to determine the problem and work to recovery. Thanks for any help offered.

Thanks to Coldcut for a very prompt reply and for the direction from his experience with my specific problem. I have inserted an image of the schematic with this comment along and have a couple more questions. Coldcut recommended replacing the outputs - are they the IC2 and IC3 (the STK-0040AT) amplifiers? And he recommended wiring across the relays - by just jumping across the contacts or should I do more? Thanks again.Thanks to Coldcut for a very prompt reply and for the direction from his experience with my specific problem. I have inserted an image of the schematic with this comment along and have a couple more questions. Coldcut recommended replacing the outputs - are they the IC2 and IC3 (the STK-0040AT) amplifiers? And he recommended wiring across the relays - by just jumping across the contacts or should I do more? Thanks again.

I do know how to check out the transistors for shorted condition and yes the STK's are the external transistors right before the switch and then to the speakers. You have have confirmed the recommendation without the schematic so unless I run into further problems, I won't upload on imageshack at this time. I will check out the transitors and replace as necessary. Thanks again.I do know how to check out the transistors for shorted condition and yes the STK's are the external transistors right before the switch and then to the speakers. You have have confirmed the recommendation without the schematic so unless I run into further problems, I won't upload on imageshack at this time. I will check out the transitors and replace as necessary. Thanks again.

Thanks for the detailed tip on the DC offset. I'll check that out and do the replacements. Thanks for sharing your high level of expertise.Thanks for the detailed tip on the DC offset. I'll check that out and do the replacements. Thanks for sharing your high level of expertise.

I see now the check won't be that direct as the STK-0040AT output transistors each are a pair of darlingtons, an NPN and a PNP. I do have an analog meter and I'll only be able to tell if I have shorted through pairs of junctions.

By the way, I received an e-mail indicating a tech buddy request came from you. I did not respond to the tech buddy request because I do not know what it entails or requires and I could not find any info about it. Can you tell me where I can find out more? Thanks for your help.I see now the check won't be that direct as the STK-0040AT output transistors each are a pair of darlingtons, an NPN and a PNP. I do have an analog meter and I'll only be able to tell if I have shorted through pairs of junctions.
By the way, I received an e-mail indicating a tech buddy request came from you. I did not respond to the tech buddy request because I do not know what it entails or requires and I could not find any info about it. Can you tell me where I can find out more? Thanks for your help.

Don't bypass the relay contacts. They serve to disconnect the amp stages from the speaker if a DC condition is present, and in doing that, save your speakers form damage. Check for a DC offset condition on the junction of the emmitter stabilizer resistors to the STK0040 on each channel. They will be 0.47 ohm power resistors(2 per STK) connected to pins 3 and 8 of the STK. Anything substantially greater that 100mv here generally indicates a blown STK. Don't bother to change both, but just the one that is faulty. Another common problem for this age of amp. The high silver content in the legs of the transistors allows for "growth" between legs and internal connections of the transistor. Check for "black death" on the legs of transistors driving the STK. . . I tend to replace them if in doubt. If you need expert advice, comment me back. I am an audio specialist.

Its very hard to check darlingtons with just a junction barrier potential check(using a resistance meter or diode checker) because not all the junctions are available at the external pins. If in doubt, just replace the STK module. They are not the expensive, and readily available, and a common cause of failure in amps that use them. Discretecomponents still has the edge over thick film hybrids for reliability, Just cheaper to make a chip that use individual parts.Its very hard to check darlingtons with just a junction barrier potential check(using a resistance meter or diode checker) because not all the junctions are available at the external pins. If in doubt, just replace the STK module. They are not the expensive, and readily available, and a common cause of failure in amps that use them. Discretecomponents still has the edge over thick film hybrids for reliability, Just cheaper to make a chip that use individual parts.

In those days I found that it was best to bypass the relays as they caused a lot of output transistors and speakers to blow.it was some stupid hot idea back then.I am still running 2 marantz amps since 78 today that are wired across the relays.the normal issue with these amps is usually always that the outputs transistors or output amps if IC are blown because of this . replace outputs and try again. use headphones while troubleshooting.If you want me to I will look at the schematic if you have one. I sold all of mine and threw a bunch away 10 years ago.if you know of one online send me a link

but if those transistors are connected through the switch to the coupling capacitor and speakers they are the usual culprits.

do you know how to measure resistance in a transistor to test if shorted.?Jumping across the contact but from under the relay is the way.
as for the schematic its too small .
stick a larger one on imageshack.
I can only guess as to the external transistors after the IC.
but if those transistors are connected through the switch to the coupling capacitor and speakers they are the usual culprits.
do you know how to measure resistance in a transistor to test if shorted.?

set your meter Rx1 , Just remove them and check for 20 ohm in one directions and open when you reverse the conection base toemitter, collector to base ,

and should show open colllector to emitter

if you see any leakage between these readings replace the xistor.

an old analog meter is best when testing xistors.

As for jumping the relay ,your choise, I always do and haven't lost anyspeakers or any more blown transistors since I have fixed over ahundred of these 30 years ago. a lot of the units are still in serviceincluding my own. when you power on the unit you have at least 5 sec toturn it off/down before it blows as you will hear the issue. with therelays working you can blow the speakers as they engage if the vol isway up . or blow out your windows as I did. I cracked 4 double panedwindows in my old house. Lol . hurt more than my pocket book onthat one.

I had verrry large speakers.set your meter Rx1 , Just remove them and check for 20 ohm
in one directions and open when you reverse the conection base to
emitter, collector to base ,
and should show open colllector to emitter
if you see any leakage between these readings replace the xistor.
an old analog meter is best when testing xistors.
As for jumping the relay ,your choise, I always do and haven't lost any
speakers or any more blown transistors since I have fixed over a
hundred of these 30 years ago. a lot of the units are still in service
including my own. when you power on the unit you have at least 5 sec to
turn it off/down before it blows as you will hear the issue. with the
relays working you can blow the speakers as they engage if the vol is
way up . or blow out your windows as I did. I cracked 4 double paned
windows in my old house. Lol . hurt more than my pocket book on
that one.
I had verrry large speakers.

Tell us some more! Your answer needs to include more details to help people.You can't post answers that contain an email address.Please enter a valid email address.The email address entered is already associated to an account.Login to postPlease use English characters only.

Looks like the amplifier output transistors have blown (hence the pop). Quite expensive too repair - I would have a look on eBay for a cheap used but working Kenwood receiver, it would probably work out cheaper.

Your unit is going into protect mode. Check your speaker wires and speakers to make sure you don't have a short. If they check good you more than likely have a channel that went out and needs rebuilt. If you have a DVM you can test your output transistors to find which chanel has the short.

It is possible that the amplifier had encountered a fault with a higher current drain which had triggered it to a protection fault condition. The fault can be with your output drivers. Please check for short on the outputs fitted for both the channels. Use a meter after disconnecting to check for short in the drivers. Disconnect the positive and negative voltages to the output and see if the protect changes. Even a fault in the preamp stages that drives in high current into the output can shut the Amplifier. Sometimes this can be a noise which can be a HUM or HISS before the protect works. Faulty capacitors in these circuits also can cause similar issues and needs close observation.You need to confirm and replace the specific stages or outputs. If not there can be issues in the mother board. Maybe the protect circuit by itself is shutting off due to a faulty bias or there is a leak in any voltage/current sensing circuit. Also disconnect the speakers and test, if the amplifier comes out of the protect mode then check for short on the speakers.

Generally speaking, an amp attempts to protect itself from heat, shorts, overloads and operator exuberance by refusing to turn on or stay on.

Overloads can be from excessive periods of high output or marginally low impedance loading by the speakers; and shorts would be wiring issues or a speaker blowing up.

You should be able to feel if it's hot. WHY is it overheating? Make sure it has sufficient ventilation on all sides and that vent holes are not blocked by dust balls. Ensure the fan (if equipped) is running as designed (some only operate on demand). Clean dust and debris from it.

If the amp comes back on after cooling, you're lucky. They only have so many self-protection cycles in their lives so continuously resetting or cycling their power without addressing the cause can do more harm than good.

If it protects immediately on a cool power up you should disconnect the speaker connections and try it 'naked'. If it comes up then diagnose which lead(s) are shorted. If it does not come up the problem is internal and should be left to an experienced and competent hands-on tech.

Check for loose speaker connections at the speaker as a root cause for intermittent shutdown.

The unit is in what is commonly known as "protect" mode. This means that a problem has been detected in the final output stage of the amp and the outputs are shutdown to prevent further damage to either the receiver or speakers. This should not be a difficult repair for a service shop. Expect a parts cost in the $25-$40 range. Add in the local labor rate for the total repair cost.

This unit is most likely in protect mode. This occurs when the receiveer detects a problem with the output amplifier section and that section is disabled. The most common failure parts are the output transistors. Expect a parts cost between $15-$40, Add in the local labor rate for the total repair cost.
Dan

I would suspect the protection circuit is bad (bad cap). It worked before??Find what is driving the speaker protection relay. Replace the small electrolytic caps in that section. Small caps are used so that when the power is removed the relay releases quickly. Or DC is present on one or more of the amps before the relay.